IPT systems typically comprise a primary conductive path (sometimes referred to as a track) which is energised by an appropriate electrical power supply such that a current is provided in the primary conductive path. The primary path will typically be an elongate cable which is energised with radio frequency (RF) current by a power supply. Distributed along the primary conductive path, but not in electrical contact with the path, are one or more pick-up devices which each have a secondary pick-up coil in which a current is induced by virtue of mutual inductance between the pick-up coil and the primary conductive path. Each pick-up usually includes control apparatus which controls the flow of power from the primary conductive path to the pick-up device so that the pick-up device may supply a load. IPT systems and an appropriate control implementation are disclosed generally in U.S. Pat. No. 5,293,308 to Boys. Furthermore, U.S. Pat. No. 6,459,218 discloses an application of IPT systems in powering road studs.
It is desirable to enable pick-ups to be controlled in various ways. In the road stud example, it is desirable to control the manner in which road studs are individually energised to enable traffic flow control. Therefore, in one example, road studs in a string may be controlled to be sequentially energised at a rate which can indicate to drivers a required speed of traffic flow.
In order to control the pick-ups so that the loads are operated in the desired fashion some form of communication is desirable between apparatus remote from the pick-up device, and the pick-up device itself. In one proposed communication system which is set out in published PCT patent specification WO 2005/031944, the primary conductive path is used as a communication path for instructions to be addressed to individual pick-up units by frequency modulating the current in the primary conductive path.